1. Field of the Invention
The invention relates in general to a stepper and a method used for an alignment of positions of a wafer stage or a wafer on the wafer stage. More particularly, the invention relates to an alignment system and method using a chopper instead of an interferometer.
2. Description of the Related Art
In a semiconductor fabrication process, photolithography process plays an important role for further reduction of dimensions of devices and higher integration of more and more devices. In the photolithography process, whether the wafer stage or the wafer on the wafer stage is precisely aligned directly determined the precision of pattern, and thus, determines the quality of devices. In a currently widely used stepper, some sub-systems such as wafer stage, alignment system, illuminator and lens determine the performance of the stepper, and consequently, determine the properties and characteristics of the end products.
A typical stepper has a high speed stage that xe2x80x9cstepsxe2x80x9d the wafer precisely with respect to the imaging optics and the IC reticles (photomask), moving the distances necessary to exactly repeat the image field in a Cartesian grid and thus fill the wafer surface. In the typical reduction stepper the stage travels in the horizontal plane underneath a fixed, vertically mounted lens. Once the wafer is placed on the exposure chuck and stepped under the lens, it is aligned by automatic systems that detect wafer targets optically and move the stage in small increments to correct the wafer position with respect to the ideal image field. The wafer is also positioned in vertical axis by autofocus systems, which in modern steppers also include the capability to pivot the vacuum chuck that holds the wafer during exposure in order to reduce any net tilt in the wafer surface due to chuck or wafer flatness errors.
FIG. 1 shows a conventional technique that employing heterodyne Interferometer to measure the position of the wafer stage. A laser beam emitted from a laser light source 102 reaches a beam splitter 108. Ideally, half portion of the laser beam passes through the beam splitter 108, while the other half portion of the laser beam is reflected to a reference mirror 106. These two beams then merge onto a detector 104. The combination of these two beams is either a result of constructive interference or a destructive interference, depending on the phase of the beams. The intensity of the signal detected by the detector 104 reflects the position of the wafer stage 100. The shifting direction of the wafer stage 100 can be obtained from the light reflected by the stage mirror according to the theory of Doppler Shift.
In addition, by installing one heterodyne interferometer at each direction such as X-axis, Y-axis and Z-axis can thus measure the displacement or rotation of the stage.
In the above conventional design of alignment system for wafer stage, the precision of the interferometers is prominently affected by vibration, stability of light source and fluctuation of power source. Especially for scanning the photomask and wafer, the control of vibration is the major factor to determine the throughput of the stepper.
The invention provides an alignment system for a wafer stage. A chopper is placed between the wafer stage and a detector. The wafer stage is placed within the scanning coverage of the chopper, while the detector measures a duty cycle and a phase of observation of the object at the other side of the chopper. According to the measured duty cycle, whether the wafer stage is aligned can be determined. This alignment system is not affected by vibration and the stability of light source or power source. Therefore, even the vibration of the wafer stage or the system is serious or the light source or power source is unstable, the position of wafer stage can be precisely corrected.
Thus, by employing the alignment system mentioned above, a method of precisely aligning a wafer stage is further provided. A wafer stage which may include an alignment mark thereon is provided. A chopper is placed on top of the wafer stage, or more specifically, on top of the alignment mark on the wafer stage, while the wafer stage of the alignment or object on the wafer stage to be aligned is within coverage of the chopper. The chopper is rotating with a constant speed, while a detector is disposed behind the chopper. The duty cycle and the phase for observing the wafer stage in front of the chopper are read by the detector. While the duty cycle and phase read from the detector are not consistent with a predetermined duty cycle when the wafer stage is precisely aligned, according to the difference of the duty cycle and phase, the exact positions of the wafer stage can be obtained.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.